1. Field of the Invention
The present invention relates to a double-ended isolated DC-DC converter, such as a half bridge DC-DC converter, a push-pull DC-DC converter, or a full bridge DC-DC converter.
2. Description of the Related Art
FIG. 1 illustrates an example of an existing double-ended isolated DC-DC converter. A double-ended isolated DC-DC converter 120 includes an external drive circuit 114, totem-pole drivers 116 and 118, a DC level shifter 121, a first switch driver 122, a second switch driver 124, primary side switches Q1 and Q2, a main transformer T1, a signal transmission transformer T2, a first synchronous rectifier Q3, a second synchronous rectifier Q4, a first switch Q5, a second switch Q6, a third switch Q7, a fourth switch Q8, a choke coil Lo, an output smoothing capacitor Co, resistors R1, R2, R3, and R4, capacitors C1, C2, C3, and C4, and diodes D1, D2, D3, and D4.
In the double-ended isolated DC-DC converter shown in FIG. 1, when a direct current voltage is applied from an input direct current power supply Vin, the primary side switches Q1 and Q2 alternately perform a switching operation. Thus, the direct current power is converted into alternating current power. The alternating current power is transmitted from a primary side circuit to a secondary side circuit of the main transformer T1 by the main transformer T1. The alternating current power is then rectified by the first synchronous rectifier Q3 and the second synchronous rectifier Q4. Thereafter, the alternating current power is smoothed by the choke coil Lo and the output smoothing capacitor Co and is converted into a direct current again. A feedback circuit (not shown) detects an output voltage and generates an error signal by comparing the output voltage with a reference voltage. The feedback circuit then transmits the error signal from the secondary side circuit to the primary side circuit. A PWM control circuit outputs first and second PWM signals. The rise of the first PWM signal is delayed by the diode D1, the resistor R1, and the capacitor C1, while the rise of the second PWM signal is delayed by the diode D2, the resistor R2, and the capacitor C2. Subsequently, the first and second PWM signals are input to the corresponding drivers. The drivers generate gate drive signals of the primary side switches Q1 and Q2 based on the input signals. At the same time, the first and second PWM signals are input to the signal transmission transformer T2 so that a combined signal of the first PWM signal appearing at a first pole of a transformer coil voltage and the second PWM signal appearing at a second pole of a transformer coil voltage is generated. The synthesis signal is transmitted to the secondary side circuit. The transmitted synthesis signal is applied to the DC level shifter 121. Thus, the DC levels of driving voltages of the totem-pole drivers 116 and 118 are increased. Accordingly, the ON period of the totem-pole drivers 116 and 118 is increased. As a result, the first synchronous rectifier Q3 and the second synchronous rectifier Q4 are driven at a timing substantially complementary to that of the primary side switches Q1 and Q2.
A conventional double-ended isolated DC-DC converter similar to that shown in FIG. 1 is disclosed in Japanese Unexamined Patent Application Publication No. 2003-511004.
In the existing double-ended isolated DC-DC converter shown in FIG. 1, since the first synchronous rectifier Q3 and the second synchronous rectifier Q4 are driven at a timing substantially complementary to that of the primary side switches Q1 and Q2, there is no period of time for a secondary coil output current of the main transformer T1 to flow through a parasitic diode of the synchronous rectifier. In addition, a short-circuited current caused by a shift of a synchronous rectifier driving timing is not generated. Accordingly, a highly efficient power conversion operation can be provided.
However, in the existing double-ended isolated DC-DC converter shown in FIG. 1, since the first and second PWM signals are combined in the primary side circuit and are separated in the secondary side circuit, a combining/separating circuit is required. Accordingly, the circuit configuration is disadvantageously complicated. Since the signal transmission transformer T2 needs to transmit a signal at a switching frequency (several tens of kHz), a relatively high inductance of 100 μH or more is required, for example. Therefore, the size of the signal transmission transformer T2 is increased. Thus, it is difficult to reduce the size and weight of the converter.